Hand held electronic data processing devices for receipt, storage and processing of information are used increasingly in connection, for example, with inventory control in retail outlets. Such devices often include electronic apparatus which maybe adversely affected by loss of power. Certain solid state memories, for example, are "volatile" in the sense that data stored therein will be lost if power is removed from the memory. Devices using these memories therefore require an uninterrupted source of electrical power to preserve the information stored therein.
Power for such devices is usually provided by a battery pack which includes one or more dry d.c. cells of the rechargeable, or nonrechargeable type. In either case, replacement of the cells is eventually necessary. Should the need for replacement of the battery or batteries occur, for example, during an inventory, removal of the battery pack for as much as a few seconds could result in the loss of all the data stored in the memory and the loss of the hours or days of time required to collect and store the information in the processor. The battery pack is often carried in a removable container, or retained in the housing by a removable member, e.g., a top or cover plate.
In order to prevent loss of stored information during battery replacement, some prior devices have included electronic means such as a charged condenser to maintain power on the memory. Due to the limited power storage capacity of the condensor, however, this may add as little as a few seconds of data retention after removal of the batteries, or other power source. If one battery is inadvertently put into a pack of several batteries backwards, for example, the time remaining to discover and correct the problem may exceed that available, particularly if power dissipation is high, due to the inclusion of a substantial amount of solid state memory.
One method of alleviating this problem has been to provide a secondary source of power in parallel with the primary source of power. The secondary source is engaged temporarily while replacement of the primary batteries is effected. This is effective unless the secondary battery is itself discharged or inadvertently removed at the same time the primary source of power is removed. In either case, the stored information may be lost.